Ion-conductive polymer electrolytes are known. For example, the following five classes of polymer electrolytes have been disclosed: (1) organic polymer electrolytes of polyethylene oxide (PEO); (2) organic polymer electrolytes produced by doping organic compounds containing a random copolymer of polyethylene oxide and polypropylene oxide in a polyfunctional polyether, with electrolytic salt, and crosslinking the compounds, as disclosed in Japanese Provisional Patent Publication No. SHO-62-249361; (3) solid polymer electrolytes comprising ethylene oxide copolymers containing ionic compounds in a dissolved stated, as disclosed in Japanese Patent Publication No. SHO-61-83249; (4) ion-conductive polymer electrolytes using plastic polymer solid materials comprising essentially thermoplastic homopolymers having no intersecting bonding (e.g., crosslinking) or branch chains of copolymers, as disclosed in Japanese Patent Publication No. SHO-55-98480; and (5) organic polymer electrolytes produced by crosslinking organic compounds having a multifunctional polyether molecular structure after doping said compounds with electrolytes, said polyether molecular structure having side chains having polyether structure, as disclosed in Provisional Patent Publication No. HEI-3-200865.
However, when the conventional ion-conductive polymer electrolytes of the above-described classes (1) through (4) were used in practical applications, for example, as electrolytes for batteries, the ion-conductive polymer electrolytes demonstrated the serious disadvantage of insufficient ionic conductivity. To solve the problem of insufficient conductivity, some investigators attempted to impregnate the conductive polymers with nonaqueous solvents, such as propylene carbonate. The nonaqueous solvents were used in conventional liquid electrolytes for the purpose of improving the conductivity, as disclosed in Japanese Provisional Patent Publication No. SHO-63-94501. The conductivities of the solvent-impregnated solid polymer electrolytes thus obtained approach a sufficient level of conductivity for practical use. Such solvent-impregnated solid polymer electrolytes, however, have the disadvantage that, when the polymer is used at an elevated temperature (i.e., 60.degree. or greater), the solvent evaporates and the performance of the solid polymer electrolytes deteriorates significantly. Moreover, the risk exists in sealed batteries, such as those using metallic lithium, that the nonaqueous solvent can damage the sealing system. Therefore, it is not possible use solvent-impregnated solid polymer electrolytes in large-sized batteries that operate at elevated temperatures.
The crosslinked polymers of above-described class (2) do not flow at relatively high temperatures and have excellent mechanical properties. The crosslinking, however, restrains the movement of segments in their molecular chains. As a result, the ionic conductivity is at most 10.sup.-4 S/cm (Siemens per centimeter) at 80.degree. C. Such ionic conductivities are inadequate for practical applications.
The thermoplastic polymers of the above-described class (4) generally have a higher ionic conductivity relative to the crosslinked polymers of class (2). The thermoplastic polymers however have the disadvantage of a tendency to flow at high temperatures. The conventional ion-conductive polymer electrolytes of above-described classes (1) through (4), therefore, are not satisfactory as electrolytes in large-sized batteries or the like that operate at relatively high temperatures (e.g., 60.degree. C. to 80.degree. C.), such as batteries used for load levelling and in electric vehicles.
The polymer electrolytes of above-described class (5) are crosslinked, and accordingly do not flow at high temperatures. The polymer electrolytes of class (5) have higher ion conductivities relative to the polymer electrolytes of class (2) and are useful in practical applications. However, the polymer electrolytes of class (5) still need much higher conductivities for many applications, such as in batteries.
In view of the above-described disadvantages and drawbacks demonstrated by the prior polymer electrolytes, investigations were conducted to provide ion-conductive polymer electrolytes that have sufficiently high ionic conductivities for use in a variety of practical applications and that are safe even at relatively high temperatures. The present invention is directed to such ion-conductive polymer electrolytes.